A variety of procedures are available for the production of simple and polymeric ester products. These include direct synthesis by reacting an organic alcohol and carboxylic acid, and interchange reactions where an acyl ##STR1## moiety or alkoxyl (--OR) moiety of an existing ester is exchanged for a different acyl or alkoxyl moiety (see Kirk-Othmer "Encyclopedia of Chemical Technology" Volume 8, pages 356-365, John Wiley & Sons, New York, 1965). Such reactions include acidolysis (reaction of the ester with a carboxylic acid to exchange acyl moieties) and alcoholysis (reaction of the ester with an alcohol to exchange alkoxyl moieties). In transesterification reactions, two ester products are heated to bring about interchange of both the acyl and alkoxyl groups as follows: ##STR2## The ester interchange reactions are particularly useful in situations where one of the resulting products is relatively low boiling and can be removed by distillation to shift the equilibrium and drive the reaction to completion. In the preparation of polyesters, for example, this feature makes it possible to obtain high molecular weight polymers. By causing the redistribution of the acyl moieties it is also possible to modify triglycerides or similar compounds containing a plurality of ester groups to obtain a wide variety of useful products.
With the above procedures it is generally advantageous to employ catalysts. Mineral acids such as hydrochloric acid, sulfuric acid and phosphoric acid have been generally used for esterification as have other acids, such as p-toluenesulfonic acid and benzenesulfonic acid. Basic materials, notably the alkali metal alkoxides and alkali and alkaline earth metal carbonates have also been used in ester production as catalysts for alcoholysis reactions. There are, however, two major problems associated with the use of these acidic and basic catalysts-- they promote side reactions resulting in the formation of undesirable by-products and require neutralization at the completion of the reaction.
Because of these drawbacks transition metal compounds, particularly titanium compounds containing alkoxyl or acyl groups have found increasing use as catalysts for ester processes since they do not require neutralization and they minimize objectionable side reactions. It is well documented, however, that these titanium catalysts hydrolyze in the presence of water with proportionate loss of activity. This feature limits their utility and in many cases requires a large amount of catalyst to insure completion of the esterification. This adds increased cost to the process and, more importantly, makes it increasingly difficult to obtain esters free of catalyst residues. The presence of catalyst residue in the ester product is undesirable since it imparts undesirable haze or color to the ester and contributes to the oxidative, thermal and hydrolytic instability of the ester.
Condensed polytitanates of varying degrees of hydrolysis and polymeric orthotitanates derived from organic polyhydroxy compounds have been disclosed in U.S. Pat. No. 2,689,858 and German Pat. No. 1,142,868, respectively. These catalysts present a definite improvement in hydrolytic stability over alkyl orthotitanates and titanium acylates, however, the amount of catalyst residue in the ester products is still unacceptable, particularly for critical applications where sparkling clear ester products with good stability are required.